Conventionally, formation of a sign on a sheet signboard such as a backlit signboard, an awning, a truck hood or the like is achieved in such a manner that a desired color film is cut into a desired sign pattern which is in turn pasted on a base sheet. Where a pale color sign is to be formed on a dark color background, a stencil cut-away patterning method is employed in which a sign portion is removed from a color film and the resulting film is pasted on a base sheet. Further, Japanese Unexamined Patent Publication No. 5-8364 (1993) discloses a stencil etch-away patterning method in which a removable color layer is preliminarily formed on a base layer of a laminated sheet such as used for an illumination signboard (or a backlit signboard) and a desired portion of the color layer is removed with the use of a solvent for formation of a sign on the sheet. In these arts, the base sheet or layer is composed of a vinyl chloride resin composition which contains a plasticizer in a proportion of several tens percent. The plasticizer is liable to migrate to the surface of the base layer with time to adsorb dust, which contaminates the surface of the sheet. To overcome this drawback, a conventional sign sheet is laminated with a stain resistant film or coated with a stain resistant resin composition.
Japanese Unexamined Patent Publication No. 5-8364 further discloses an art in which a barrier layer is formed on the base layer for prevention of the migration of the plasticizer and the color layer (third layer) is formed on the barrier layer. This art, however, requires additional steps and time for the formation of the barrier layer. Further, when the barrier layer is formed by coating with a coating resin composition, the plasticizer contained in a great amount in the base layer may leach out into an organic solvent contained in the coating resin composition or migrate to the barrier layer. Moreover, when the resulting sheet is used at a high temperature, the liquid plasticizer is liable to migrate to the surface of the barrier layer because the barrier layer is fairly thin. Therefore, this art may lead to impairment of the stain resistant property of the sheet.